I am somewhat confused about the two Canon FD 2x teleconverters. I have seen reported that the 2x-A is recommended for lenses 300mm and longer and the 2x-B for lenses <300mm. I also have an old Canon brochure ("Canon Interchangeable Lenses" circa 1975) which describes the 2x-A as being appropriate for "any FD fixed focal length lens ranging from 100mm to 800mm or any FD zoom with 100mm in its range." This brochure does not show the 2x-B, so I assume it did not yet exist at that time. Why the discrepancy regarding the 2x-A? Did Canon change the optical formula for the 2x-A when the 2x-B was introduced? Any info would be much appreciated.